convert the mesh to a brush and then apply blocking volume to brush. I did that with the SPHAT in Anchorhead. Otherwise you'll never get it to fit properly. You may have to replace the model after, I can't remember for sure

Select the Static Mesh, right click on it -> Convert to -> Brush.Select both the Static Mesh and the Brush, right click -> Save Brush as Collision.Make sure only to do this with meshes you've created or imported yourself, or save the Static Mesh under another name in one of your custom Mesh folders first.